ESLint 6.2.0 + babel-eslint + no-unused-vars false positive with for-in loop

There’s a new issue in ESLint 6.2.0 just caught by the standard test suite. Issue did not exist in ESLint 6.1.0.

Tell us about your environment

• ESLint Version: 6.2.0
• Node Version: 10.16.3
• npm Version: 6.9.0

What parser (default, Babel-ESLint, etc.) are you using? babel-eslint

Please show your full configuration:

Configuration

{
  "parserOptions": {
    "ecmaVersion": 2019,
    "ecmaFeatures": {
      "jsx": true
    },
    "sourceType": "module"
  },
  "parser": "babel-eslint",
  "rules": {
    "no-unused-vars": ["error", { "vars": "all", "args": "none", "ignoreRestSiblings": true }]
  }
}

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

function scoreAudioCoverFile (imgFile) {
  const fileName = path.basename(imgFile.name, path.extname(imgFile.name)).toLowerCase()
  const relevanceScore = {
    cover: 80,
    folder: 80,
    album: 80,
    front: 80,
    back: 20,
    spectrogram: -80
  }

  for (const keyword in relevanceScore) {
    if (fileName === keyword) {
      return relevanceScore[keyword]
    }
    if (fileName.indexOf(keyword) !== -1) {
      return relevanceScore[keyword]
    }
  }
  return 0
}

npx eslint --config eslintrc.json t.js

What did you expect to happen?

Just one error:

  1:10  error  'scoreAudioCoverFile' is defined but never used  no-unused-vars

What actually happened? Please include the actual, raw output from ESLint.

Two errors:

   1:10  error  'scoreAudioCoverFile' is defined but never used  no-unused-vars
  12:14  error  'keyword' is defined but never used              no-unused-vars

The variable keyword from the for-in loop is seen as not used.

This may be a bug in babel-eslint since the issue does not happen when the default parser is used. However, this issue did not exist with babel-eslint + 6.1.0 and does exist with babel-eslint + 6.2.0.

Issue opened on babel-eslint: https://github.com/babel/babel-eslint/issues/791

Are you willing to submit a pull request to fix this bug? Yes

[yoyo837]

https://github.com/babel/babel-eslint/pull/794 fixed this.

[vincerubinetti]

To whom it affects:

As of yesterday (I think), create-react-app finally released its fix for this babel-eslint bug in react-scripts version 3.1.2.

If you were doing any dirty hacks to temporarily get around this, now’s the time you can finally remove them.